One of methods for joining two steel members is friction welding that involves bringing joining surfaces of these members into abutment with each other and relatively rotating the members to generate frictional heat whereby softening the joining surfaces, and applying high pressurizing force to the members to form diffusion bonding between the members. Friction welding is employed to manufacture various components, such as propeller shafts for automobiles.
The steel material for friction welding is often required not only to have high friction weldability, but also to have good machinability. For example, the propeller shafts described above are manufactured being subjected to not only friction welding but also machining.
It has been well known that addition of Pb is effective for improvements in the machinability of steel. In contrast, environmental concerns demand Pb-free steel without Pb addition. The addition of S in place of Pb can provide steel with improved machinability; however, which machinability is barely compatible with the friction weldability.
Patent literature 1 discloses a Pb-free steel material for machine structures suitable for friction welding, wherein N is preliminarily present in the form of a compound while the content of N in solid solution is regulated to 0.0015% or less, and the number of MnS grains with a certain size is controlled. This patent further describes that, for the regulation of the content of N in solid solution and the control of the MnS grains, a billet should be heated at a temperature in the range of 1000 to 1250° C. and the temperature should be maintained within the range of 800 to 1000° C. from the start to the end of the hot rolling (Paragraph 0091).
Furthermore, the patent describes that a change in existing form of N present in the steel before and after the friction welding can improve the joining strength after the friction welding, cold workability, and machinability (Paragraph 0029).